The present invention relates to an optical recording element which is capable of recording and retrieving information by means of laser beam radiation or the like, and the manufacturing method thereof.
A conventional optical recording element such as a video disk has a structure as shown in either FIG. 1 or FIG. 2. In FIG. 1, it consists of a recording layer 2 which is vacuum deposited onto a substrate 1, and a protective layer 3 which is formed by coating an organic material solution on the recording layer 2 by dipping or by means of a spinner, an applicator, or the like, followed by curing. In FIG. 2, it comprises a recording layer 6 which is vacuum deposited onto a substrate 5, and a protective layer 8 which is bonded to the recording layer 6 with an adhesive 7. Such a protective layer protects the recording layer from dust and damage, and prevents the condensing lens from being contaminated by scattered recording material when information pits are recorded.
As shown in FIG. 3, recording and retrieving of information with the optical recording element is done by a beam 11 which is focused on a recording layer 15, for example 1 to 1.5 .mu.m in diameter, using a converging lens 10. When the recording layer 15 is irradiated with the recording beam in response to a recording signal, the recording material of only the irradiated portion of the recording layer 15 evaporates or melts to form a record pit 16. Upon retrieval, the intensity of the retrieval beam reflected from the record pit 16 is markedly different from that reflected by other spots that lack the record pit 16. Accordingly, the signal can be retrieved by detection of variations in the intensity of the reflected beam caused by the presence or absence of the record pit 16. A detector 13 and a beam splitter 12 may be used for this purpose.
In such recording and retrieving, dust 18 and scratch 19 on the surface of the protective layer 17 are important causes of imprecise recording and retrieval. If dust of about 20 .mu.m diameter, which represents the greatest proportion in a general environment such as an office room, is deposited on the protective layer surface, the beam-focusing diameter on the protective layer surface has to be made so large that the influence of dust of this size becomes negligible. According to the description of IEEE vol. CE-20, No. 9 (1976), pp. 309-316, the thickness of the protective layer must be at least 200 .mu.m if the influence of dust of 20 .mu.m in diameter is to be negligible. Since forming such a thick protective layer by vacuum deposition takes quite a long time, it is formed either by coating and curing of an organic material solution as shown in FIG. 1, or by bonding a light-transmitting film to the recording layer as shown in FIG. 2.
Such conventional methods, however, present difficulties in completely removing dust, foreign substance and bubbles from the organic material solution or the adhesive. Also, dust and bubbles are apt to be included during formation of the protection layer, so that special apparatus or equipment is required to prevent these. Because the dust and bubbles are located either within the protective layer or between the protective layer and the recording layer where the beam is incident, the beam will be scattered by hitting these particles during recording or retrieving, making recording or retrieving impossible or resulting in a noise increase, hence, a degradation in SN ratio, or still worse, disabling retrieval.
In FIGS. 1 and 2, examples having only a recording layer between the substrate and protective layer are shown, but in cases where another intermediate layer is included in addition to the recording layer, such as a silicon dioxide layer or an aluminum layer for the purposes of heat insulation, improvements of the retrieval SN ratio and recording sensitivity or the like, the aforementioned drawbacks are also involved.